Diffuser with non-constant diffuser vane pitch and centrifugal turbine including said diffuser

Abstract

A novel diffuser (11) for a centrifugal turbine (1) to reduce stall or prevent the occurrence of stall. The diffuser includes diffuser vanes (11.1) arranged around a diffuser axis (A-A). Each diffuser vane (11.1) includes: a leading edge (11.3), a trailing edge (11.5), a radially inward-facing suction side (11.7), and a radially outward-facing pressure side (11.9). A corresponding flow passage is defined between the suction side (11.7) of a first diffuser vane (11.1) and the pressure side (11.9) of a second diffuser vane (11.1) in each pair of adjacent diffuser vanes arranged. The diffuser vanes (11.1) are arranged around the diffuser axis (A-A) with a non-constant pitch. The pitch (S1, S2) between a first diffuser vane (11.1) and a second diffuser vane (11.1) in each pair of adjacent diffuser vanes, between which a corresponding flow passage is defined, is related to the chord of one of the first diffuser vane (11.1) and the second diffuser vane (11.1).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to radial turbines. More specifically, embodiments of the present disclosure relate to centrifugal turbines, such as centrifugal pumps and / or centrifugal compressors including one or more novel vane diffusers (i.e., blade diffusers). BACKGROUND OF THE INVENTION

[0002] Centrifugal compressors are used in a variety of applications to increase gas pressure. A centrifugal compressor includes a housing and one or more impellers arranged to rotate within the housing. Mechanical energy transferred to the impeller is transferred to the gas in the form of kinetic energy by rotating the impeller. The gas accelerated by the impeller flows through a diffuser circumferentially surrounding the impeller, which collects the gas flow and reduces its velocity, converting the kinetic energy into gas pressure.

[0003] To better direct the gas flow through the diffuser, vane diffusers have been developed. The diffuser vanes redirect the gas flow in a more radial direction and improve the aerodynamic efficiency of the compressor. However, the diffuser vanes generate pressure pulses that excite the vibration of the impeller blades. Impeller vibration can cause impeller failure due to high cycle fatigue (HCF).

[0004] To reduce the risk of impeller failure caused by diffuser vane-induced vibration, centrifugal compressors with so-called aperiodic diffusers have been developed. An aperiodic diffuser is a vane diffuser in which the diffuser vanes are arranged in an asymmetric and aperiodic arrangement. For example, aperiodic diffusers for centrifugal compressors are disclosed in US 7,845,900 and WO2011 / 096981.

[0005] Some embodiments of aperiodic diffusers for centrifugal compressors include diffuser vanes arranged according to a variable pitch, i.e., arranged such that the angular spacing between two adjacent diffuser vanes defining a flow passage therebetween is different from the angular spacing between two other adjacent diffuser vanes defining another flow passage therebetween. It has been found that the irregular (i.e., non-constant) angular spacing of the diffuser vanes reduces the excitation of impeller blade vibration.

[0006] However, the asymmetric and non-periodic design of the diffuser vanes has an adverse effect on the operating range of the compressor. More specifically, increasing the angular spacing (pitch) between adjacent diffuser vanes results in a decrease in the density of the associated flow passage. The density is the ratio between the chord of the vane (i.e., the distance between the trailing edge and the leading edge of the vane) and the pitch between two consecutive vanes. The reduced density results in a narrowing of the mass flow range within which the compressor can operate without stalling or suffering a significant reduction in performance. The minimum mass flow rate at which stall conditions are reached increases due to the reduced density. Thus, although beneficial in terms of reducing vibrations, variable vane pitch is detrimental when the operability of the compressor is reduced.

[0007] The art would welcome a new diffuser design that improves the performance of the compressor in terms of reducing impeller vibrations while having less of a negative impact on the operating range of the compressor. Summary of the Invention

[0008] According to one aspect of the present disclosure, a diffuser for a centrifugal turbine, such as a centrifugal compressor (or centrifugal pump), is provided. The diffuser includes a plurality of diffuser vanes circumferentially arranged about a diffuser axis. Each diffuser vane includes a leading edge spaced a first distance from the diffuser axis, a trailing edge spaced a second distance from the diffuser axis, a suction side facing radially inward and extending from the leading edge to the trailing edge, and a pressure side facing radially outward and extending from the leading edge to the trailing edge, the second distance being greater than the first distance. The diffuser vanes define a plurality of flow passages. More specifically, between each pair of adjacent (i.e., consecutive) vanes, a flow passage is defined between the suction side of a first diffuser vane and the pressure side of a second diffuser vane in each pair of diffuser vanes. The diffuser vanes are arranged about the diffuser axis with a non-constant pitch. To improve the operating range of the compressor and reduce the negative impact that pitch variations have on the operability of the compressor, the pitch between each pair of adjacent arranged first and second diffuser vanes that define a corresponding flow passage therebetween is related to the chord (specifically, the length of the chord) of one of the first and second diffuser vanes.

[0009] More specifically, the chord related to the pitch is the chord of the diffuser vane whose suction side faces the flow passage.

[0010] The correlation between the chord and the pitch causes the reduction in density that might otherwise be caused by an increase in the pitch between the diffuser vanes to be at least partially offset by an increase in the chord length.

[0011] The present disclosure also discloses a vaned diffuser for a centrifugal turbine (in particular, a centrifugal compressor (or a centrifugal pump)) including a plurality of diffuser vanes circumferentially arranged around a diffuser axis. Each diffuser vane includes: a leading edge, a trailing edge, a suction side that faces radially inward and extends from the leading edge to the trailing edge, and a pressure side that faces radially outward and extends from the leading edge to the trailing edge. A corresponding flow passage is defined between the suction side of the first diffuser vane and the pressure side of the second diffuser vane in each pair of adjacent diffuser vanes arranged adjacent to each other. The diffuser vanes are arranged around the diffuser axis with a non-constant pitch. In addition, the diffuser vanes have a non-constant chord, and the ratio between the chord of the first diffuser vane and the pitch between the first diffuser vane and the second diffuser vane in each pair of diffuser vanes is substantially constant.

[0012] The diffuser vanes can be arranged such that the leading edges of all the diffuser vanes are arranged on the same circumference around the diffuser axis. In such a case, the pitch between adjacent diffuser vanes (between which a corresponding flow passage is formed) is the distance along the circumference between the two leading edges of the two diffuser vanes forming the flow passage.

[0013] However, as will be described in more detail in the following description of the embodiments, the diffuser vanes can be arranged such that the leading edges are not all placed on the same circumference of the smallest diameter around the diffuser axis. Instead, the two diffuser vanes of at least one pair of diffuser vanes forming a flow passage can be arranged such that the corresponding leading edges are at variable distances from the diffuser axis.

[0014] The pitch between adjacent (i.e., consecutive) diffuser vanes can be defined as the distance measured at the minimum distance from the diffuser axis between the arcs of the two adjacent diffuser vanes, where both of the two diffuser vanes are present.

[0015] The present disclosure discloses a turbine, and specifically a centrifugal compressor or a centrifugal pump, including at least one impeller and at least one vaned diffuser as defined above and below.

[0016] Additional features and embodiments of the novel diffuser and the centrifugal turbine including the diffuser are outlined below and recited in the appended claims, which form an integral part of the specification. Description of the Drawings

[0017] When considered in conjunction with the accompanying drawings, a more complete understanding of the disclosed embodiments of the present invention and many of its attendant advantages will be readily obtained by reference to the following detailed description, which will also become better understood, wherein:

[0018] Figure 1 A schematic cross-sectional view of a compressor is shown according to a plane containing the rotational axis of the compressor;

[0019] Figure 2 Shows in one embodiment a diffuser of a compressor according to Figure 1 in a cross-sectional view taken along line II-II; Figure 1 in

[0020] Figure 3 Shows Figure 1 an isometric view of a diffuser of a compressor;

[0021] Figure 4 Shows Figure 2 an enlarged detail;

[0022] Figure 5 Schematically shows the characteristic operating curve of a compressor stage in a mass flow versus pressure ratio diagram;

[0023] Figure 6 Shows Figure 5 the flow direction at two different operating points;

[0024] Figure 7 , Figure 8 and Figure 9 show the variation of pitch, chord, and solidity in diffusers according to the present disclosure in three embodiments; and

[0025] Figure 10 Shows in another embodiment a diffuser of a compressor according to Figure 1 in a cross-sectional view taken along line II-II; Figure 1 in Detailed Description

[0026] It has been found that the negative impact on compressor operability due to an increase in pitch between adjacent diffuser vanes defining the flow passage of the diffuser can be offset by a corresponding increase in the chord length of the diffuser vanes, with the suction side of the diffuser vanes facing the flow passage. In this way, the reduction in solidity caused by the increase in pitch is reduced and is at least partially offset by the corresponding change in chord. In some embodiments, the combination of pitch and chord changes can cause the solidity around the diffuser (i.e., in the various flow passages defined between adjacent vane pairs of the vane diffuser) to remain substantially constant.

[0027] Now referring to Figure 1 , a portion of a centrifugal compressor 1 is shown in a cross-sectional view along a plane containing the rotational axis of the compressor. Figure 1The part shown in [description] is limited to the first stage of a centrifugal compressor. Depending on the compressor design and compressor requirements, the number of compressor stages, and thus the number of impellers, can vary for different compressors. The novel features of the diffuser according to the present disclosure can be embodied in one diffuser, some diffusers, or preferably all diffusers of a given compressor.

[0028] The compressor includes a housing 3 in which a diaphragm 5 is arranged to separate successive compressor stages. Each compressor stage includes an impeller 7 that is supported for rotation in the housing 3. The impeller 7 can be shrink-fitted onto a rotating shaft 9. In other embodiments not shown, the impeller 7 can be a stacked impeller according to designs known to those skilled in the art of centrifugal compressors, and it is not disclosed herein. The impeller 7 has an impeller hub 7.1 from which a plurality of impeller blades 7.3 project. Each impeller blade 7.3 has a leading edge 7.5 and a trailing edge 7.7. The leading edge 7.5 is arranged along the impeller inlet, and the trailing edge 7.7 is arranged along the impeller outlet. The trailing edge 7.7 is arranged at a distance from the rotational axis A - A greater than that of the leading edge 7.5.

[0029] In Figure 1 the embodiment shown, the impeller 7 further includes a shroud 7.9. In other embodiments not shown, the impeller 7 can be an impeller without a shroud, in which case the shroud 7.9 is omitted.

[0030] A diffuser 11 is arranged around the impeller outlet. The diffuser 11 surrounds the impeller 7 and is coaxial therewith. In a cross-sectional view taken along Figure 1 the line II - II of Figure 2 and in an isometric view of Figure 3 the diffuser 11 is shown separately. In Figure 4 an enlarged view of the details shown in Figure 2 is shown. The diffuser 11 extends circumferentially around the impeller 7 and has an axis that coincides with the rotational axis A - A of the shaft 9.

[0031] The diffuser 11 is a so-called vaned diffuser provided with a plurality of diffuser vanes 11.1 arranged around the diffuser axis A - A. The purpose of the diffuser vanes 11.1 is to redirect the incoming gas flow in a more radial direction, i.e., to reduce the tangential velocity component of the gas flow exiting the diffuser 11 and to improve pressure recovery and overall stage efficiency.

[0032] Each diffuser vane 11.1 includes a leading edge 11.3 and a trailing edge 11.5. The distance between the leading edge 11.3 and the trailing edge 11.5 is referred to as the chord B of the diffuser vane 11.1. The distance of the leading edge 11.3 from the axis A - A is less than that of the trailing edge 11.5.

[0033] Each diffuser vane 11.1 further includes a suction side 11.7 and a pressure side 11.9. The aerodynamic load on each diffuser vane 11.1 causes the suction side to be the vane side facing the inlet of the diffuser 11, i.e., the radially inward-facing side of the diffuser vane 11.1. In contrast, the pressure side is the radially outward-facing side of the diffuser vane 11.1 facing the outlet of the diffuser 11.

[0034] The gas flow direction at the inlet of the diffuser 11 depends on the mass flow rate through the compressor. A more radial flow direction (lower tangential velocity component) occurs at higher mass flow rates, and a more tangential flow direction (higher tangential velocity component) occurs at lower mass flow rates. The pressure ratio of the entire compressor stage increases as the mass flow rate decreases.

[0035] Figure 5 The characteristic curve of the centrifugal compressor stage is schematically shown in a mass flow rate versus pressure ratio diagram. The mass flow rate is plotted on the horizontal axis, and the pressure ratio is plotted on the vertical axis. The characteristic curve is labeled CC. The flow angle at the diffuser inlet (i.e., the direction of the gas velocity at the inlet of the diffuser 11) becomes more tangential as the mass flow rate decreases. Figure 6 The flow angles at two opposite operating points PA and PB of the characteristic curve are schematically shown. VA and VB are the velocity vectors at the leading edge of the diffuser vane 11.1 corresponding to the operating points PA and PB, respectively.

[0036] The mass flow rate of the compressor has a lower limit that causes a stall condition. This limit is represented as the stall limit SL in the Figure 5 chart. The diffuser vane 11.1 stalls mainly on the suction side 11.7. When the velocity vector reaches the inclination of the vector VB, the flow detaches from the suction side 11.7 of the diffuser vane 11.1. To prevent damage to the compressor, the operating point of the compressor should be maintained at a safe distance from the stall limit SL.

[0037] If the solidity of the diffuser is reduced, the stall limit SL can shift to the Figure 5 right side of the chart, thereby reducing the operating range of the compressor in terms of mass flow rate. Solidity is defined as the ratio of the chord of the diffuser vane 11.1 to the spacing between two consecutive (i.e., adjacent) diffuser vanes 11.1. In a vane diffuser with a constant pitch between the diffuser vanes, the solidity is defined as

[0038]

[0039] And for each flow channel, the compactness is the same. B is the chord of the diffuser vane, and S is the pitch, i.e., the spacing between adjacent diffuser vanes 11.1, i.e., the distance between two consecutively arranged diffuser vanes 11.1.

[0040] The compactness affects the stall limit because a lower compactness may imply an earlier stall, i.e., Figure 5 the stall limit in the graph of

[0041] In a vane diffuser with non-constant pitch between circumferentially arranged diffuser vanes 11.1 of the current art, the compactness is again defined as

[0042]

[0043] For each i-th flow channel, where Si is the spacing, i.e., the pitch between two consecutive diffuser vanes 11.1 defining the i-th flow channel. Since the compactness around the diffuser is non-constant, a stall condition may occur at the flow channel with the lowest compactness (i.e., the largest pitch Si). For a compressor operating in a safe condition, the operating point should be at a safe distance from the stall limit of the most critical flow channel (i.e., the flow channel with the largest pitch). This substantially reduces the operability range of the compressor. Thus, according to the compressor design of the prior art, vibration reduction aimed at reducing the risk of high-cycle fatigue failure of the impeller reduces the operability of the compressor.

[0044] To mitigate the above disadvantages, embodiments of the present disclosure provide a novel method in diffuser design. The reduction in compactness determined by the increased pitch between adjacent diffuser vanes 11.1 is balanced by increasing the chord of the relevant diffuser vane (more specifically, the chord of the diffuser vane 11.1 where stall may occur on its suction side). Such a diffuser vane is a diffuser vane whose suction side faces the relevant flow channel.

[0045] Referring to Figure 4 and continuing to refer to Figure 1 、 Figure 2 and Figure 3 , without any general loss, an enlarged view of a part of the diffuser 11 is shown. In the present embodiment, the diffuser vanes 11.1 are arranged according to two different pitches or spacings S1 and S2. More specifically, the spacing S2 is greater than S1.

[0046] More specifically, in the present embodiment, pairs of consecutive diffuser vanes 11.1 are arranged alternately with spacings S1 and S2. In other words, moving in a clockwise direction around the diffuser axis, following a first passage P1 having a spacing S1 between the diffuser vanes 11.1 defining it is a second passage P2 having a spacing S2 (S2 > S1) between the corresponding diffuser vanes 11.1 defining the second passage P2. The next passage again has a spacing S1, and so on. In the present embodiment, the passages P1, P2 have a non-constant pitch.

[0047] If the chords B of three subsequently arranged vanes forming the passages P1 and P2 are equal, the solidity of the first passage P1 will be higher than the solidity of the second passage P2, as follows:

[0048]

[0049] wherein,

[0050] Si is the pitch or spacing of the ith flow passage

[0051] σ Pi is the solidity of the ith flow passage Pi.

[0052] The passage P2 with a lower solidity may cause stalling to occur earlier. Then, P2 will be the limiting passage for the compressor operability. To avoid this, the embodiments disclosed herein provide diffuser vanes 11.1 with a variable (i.e., non-constant) chord B. More specifically, the chord B of the diffuser vanes 11.1 is related to the pitch (i.e., the spacing S between consecutive or adjacent diffuser vanes 11.1), and increasing the chord B of one of the diffuser vanes forming the passage P rebalances the solidity of the passage, as shown below:

[0053]

[0054] wherein, Bi is the chord of one of the two diffuser vanes 11.1 defining the ith passage Pi. More specifically, Bi is the chord of the diffuser vane whose suction side 11.7 faces the ith passage Pi, as Figure 4 shown. In this case, the solidity of the diffuser flow passage is defined as the ratio of the chord of the diffuser vane whose suction side faces the flow passage to the pitch between the two diffuser vanes that define the flow passage.

[0055] By making the chord B of the first diffuser vane 11.1 of each ith flow passage Pi depend on the pitch or spacing Si between the two diffuser vanes forming the flow channel, the effect of the solidity change caused by the pitch change is balanced by the chord change.

[0056] Thus, by balancing the reduction in density due to the increase in pitch (wherein the chord of the associated diffuser vane 11.1 increases), the beneficial effect of pitch variation in reducing impeller vibration is achieved without negatively affecting the compressor operability.

[0057] In a preferred embodiment, the relationship between the chord Bi of each diffuser vane and the blade pitch or spacing Si of each ith flow passage Pi is such that the density σ of the flow passage Pi remains constant.

[0058] However, a strictly constant density value is not mandatory. Beneficial effects can also be achieved in terms of enhanced compressor operability if the density remains substantially constant in the vicinity of a preset value. As used herein, "substantially constant" can be understood as a density within a range of + / - 20% of the vicinity of a preset constant density value. According to the embodiments disclosed herein, "substantially constant" can be understood as maintaining a density within a range of + / - 10% (and preferably within a range of + / - 5%, and preferably within a range of + / - 2%) of the vicinity of a preset constant density value.

[0059] Figure 7 The figure shows a graph of pitch (spacing) S and chord B plotted against the angular position of the flow passage on the abscissa. The pitches of successively arranged pairs of diffuser vanes are labeled S1, S2,... Si,... Sn. The corresponding chords of the first diffuser vane 11.1 of each flow passage P1, P2,... Pi,... Pn are labeled B1, B2,... Bi,... Bn. The horizontal line σconst represents the constant density value, while σmin and σmax represent the minimum and maximum values of the allowable range of density values in the vicinity of the preset constant density value σconst. As described above, σmin can be 20% lower than σconst, or preferably 10% lower, or more preferably 5% lower, or even more preferably 2% lower than σconst. Similarly, σmax can be 20% higher than σconst, preferably 10% higher, or more preferably 5% higher, or even more preferably 2% higher than σconst.

[0060] In Figure 2 , Figure 4 , a cyclic variation of the pitch S between adjacent diffuser vanes 11.1 according to two different pitches S1 and S2 and the corresponding cyclic variation of the blade chord B are shown. In other embodiments, the blades can be arranged according to more than two different pitches or spacings S1, S2 ( Figure 7 ).

[0061] In other embodiments, the variations of both the spacing and the chord can be random (as Figure 8as shown) rather than cyclic. Figure 10 A cross-sectional view of a diffuser 11 showing diffuser vanes 11.1 arranged randomly.

[0062] In yet another embodiment, the variation can be monotonic, i.e., the pitch and chord can gradually decrease around the diffuser axis A-A from the first flow passage to the last diffuser passage, as Figure 9 shown.

[0063] To further reduce the vibration of the impeller blades, additional features of the diffuser vanes can vary around the diffuser axis. According to some embodiments, for example, the diffuser vanes 11.1 can have a variable profile. In some embodiments, the diffuser vanes can have a leading edge and / or a trailing edge with a variable radial position. Additionally or alternatively, the diffuser vanes can have a variable inclination.

[0064] In addition, although in Figure 1 the diffuser has a constant height, in some embodiments, the diffuser can have a variable height in the tangential direction and / or the flow direction.

[0065] The above embodiments specifically refer to a centrifugal compressor. However, the novel diffuser according to the present disclosure can also be advantageously used in a centrifugal pump, the structure of which is similar to Figure 1 the structure shown in.

[0066] Exemplary embodiments have been disclosed above and illustrated in the drawings. Those skilled in the art will understand that various changes, omissions, and additions can be made to the specific disclosure herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A diffuser (11) for a centrifugal turbine (1), the diffuser comprising: A plurality of diffuser vanes (11.1), the plurality of diffuser vanes being circumferentially arranged around a diffuser axis (A-A); wherein each diffuser vane (11.1) includes: a leading edge (11.3) at a first distance from the diffuser axis (A-A), a trailing edge (11.5) at a second distance from the diffuser axis (A-A), a suction side (11.7) facing radially inwards and extending from the leading edge (11.3) to the trailing edge (11.5), and a pressure side (11.9) facing radially outwards and extending from the leading edge (11.3) to the trailing edge (11.5), the second distance being greater than the first distance; wherein a respective flow passage is defined between the suction side (11.7) of a first diffuser vane (11.1) and the pressure side (11.9) of a second diffuser vane (11.1) in each pair of adjacent diffuser vanes arranged; and wherein the diffuser vanes (11.1) are arranged with a non-constant pitch around the diffuser axis (A-A); wherein the pitch (S1, S2) between each pair of adjacent first diffuser vanes (11.1) and second diffuser vanes (11.1) between which a respective flow passage is defined is related to the chord (B) of one of the first diffuser vane (11.1) and the second diffuser vane (11.1); wherein the diffuser vanes (11.1) have a chord of variable length; wherein the pitch (S1, S2) between each pair of adjacent diffuser vanes (11.1) and the chord of one of the first diffuser vane (11.1) and the second diffuser vane (11.1) are selected such that the density of each flow passage (Pi) remains within a range of a constant density value, and increasing the chord (B) of one of the diffuser vanes forming the passage (P) rebalances the density of the passage; and wherein the diffuser vanes (11.1) have a leading edge (11.3) of variable radial position.

2. The diffuser (11) according to claim 1, wherein, The pitch between each pair of adjacent first diffuser vanes (11.1) and second diffuser vanes (11.1) between which a respective flow passage is defined is related to the chord of the first diffuser vane (11.1), and the suction side (11.7) of the first diffuser vane faces the respective flow passage.

3. The diffuser (11) according to claim 1 or 2, wherein, The range is -20% to +20% of the constant density value.

4. The diffuser (11) according to claim 1 or 2, wherein, The diffuser vanes (11.1) have a variable profile.

5. The diffuser (11) according to claim 1 or 2, wherein, The variations of both the pitch and the chord are random.

6. The diffuser (11) according to claim 1 or 2, wherein, The variations of the pitch and the chord are monotonic, and the pitch and the chord gradually decrease around the diffuser axis A-A from the first flow passage to the last flow passage.

7. The diffuser (11) according to claim 1 or 2, wherein, The diffuser vanes (11.1) have a trailing edge (11.5) of variable radial position.

8. The diffuser (11) according to claim 1 or 2, wherein, The diffuser vanes (11.1) have a variable inclination.

9. The diffuser (11) according to claim 1 or 2, wherein, The diffuser height is variable in at least one of the tangential direction and the flow direction.

10. The diffuser (11) according to claim 3, wherein, The range is -10% to +10% of the constant density value.

11. The diffuser (11) according to claim 10, wherein, The range is -5% to +5% of the constant density value.

12. The diffuser (11) according to claim 11, wherein, The range is -2% to +2% of the constant density value.

13. A diffuser (11) for a centrifugal turbine (1), the diffuser comprising: A plurality of diffuser vanes (11.1), the plurality of diffuser vanes being circumferentially arranged around a diffuser axis (A-A); wherein each diffuser vane (11.1) comprises: a leading edge (11.3), a trailing edge (11.5), a suction side (11.7) that faces radially inwards and extends from the leading edge to the trailing edge, and a pressure side (11.9) that faces radially outwards and extends from the leading edge (11.3) to the trailing edge (11.5); wherein a respective flow channel (P) is defined between the suction side (11.7) of a first diffuser vane (11.1) and the pressure side (11.9) of a second diffuser vane (11.1) in each pair of adjacent diffuser vanes (11.1); and wherein the diffuser vanes (11.1) are arranged around the diffuser axis (A-A) with a non-constant pitch (S1, S2); wherein: the diffuser vanes (11.1) have a non-constant chord (B); wherein the pitch (S1, S2) between each pair of adjacent diffuser vanes (11.1) and the chord of one of the first diffuser vane (11.1) and the second diffuser vane (11.1) are selected such that the density of each flow channel (Pi) is maintained within a range of a constant density value, and increasing the chord (B) of one of the diffuser vanes forming the channel (P) rebalances the density of the channel; and wherein the leading edge of the diffuser vanes (11.1) is at a variable radial distance from the diffuser axis.

14. The diffuser (11) according to claim 13, wherein, The range is maintained within a range of approximately constant density value.

15. The diffuser (11) according to claim 14, wherein, The range is -20% to +20% of the constant density value.

16. The diffuser (11) according to claim 15, wherein, The range is -10% to +10% of the constant density value.

17. The diffuser (11) according to claim 16, wherein, The range is -5% to +5% of the constant density value.

18. The diffuser (11) according to claim 17, wherein, The range is -2% to +2% of the constant density value.

19. A centrifugal turbine (1), comprising: At least one impeller (7), the at least one impeller being arranged to rotate about a rotational axis (A-A); And a diffuser (11) according to any one of the preceding claims.

20. The turbine according to claim 19, wherein, The turbine is a centrifugal compressor.

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